Ion gun for production of ion beams with particular radial current density profile

ABSTRACT

An ion gun device capable of producing circularly shaped ion beams in the keV energy range with constant radial current density across a large part of the beam diameter. The device consists of an ion source, a cylindrical extraction cup electrode and a cylindrical acceleration tube electrode, all disposed on the same axis. Positive ions are axially extracted from the ion source by the extraction electrode which is kept at a small negative potential (less than one tenth of the acceleration potential) with respect to the ion source. The extracted ions are accelerated and focused by an immersion lens which is formed by the potential distribution within and between the extraction and acceleration electrodes, the acceleration electrode being at such a negative potential with respect to the ion source that the focused ions possess the desired energy after having passed through the immersion lens. Under observance of well-defined restrictive conditions for the lay-out of several geometrical dimensions of the construction and under observance of welldefined restrictive operation conditions of the ion gun it is possible to obtain circularly shaped ion beams with an approximately trapezoidal radial current density distribution at axial target position far removed from the acceleration field.

Schulz et al.

Oct. 7, 1975 ION GUN FOR PRODUCTION OF ION BEAMS WITH PARTICULAR RADIAL CURRENT DENSITY PROFILE Inventors: Friedrich Schulz; Klaus Wittmaack,

both of Munich, Germany Assignee: Gesellschaft fur Strahlenund Umweltforschung mbI-l Munchen, Neuherberg, Germany Filed: Apr. 25, 1974 Appl. No.: 464,217

Related U.S. Application Data Continuationin-part of Set. No. 356,314, May 2, 1973, abandoned.

[30] Foreign Application Priority Data Nov. 7, i972 Germany 22544 44 [52] U.S. Cl. 313/361; 313/359; 313/153 [51] Int. Cl. H05l-l 5/00 [58] Field of Search 313/359-36] [56] References Cited UNITED STATES PATENTS 3,740,554 6/1973 Morgan 313/359 X Primary Examiner-R. V. Rolinec Assistan! Examiner Darwin R. Hostetter Attorney, Agent, or Firm-Spencer & Kaye 5 7 ABSTRACT An ion gun device capable of producing circularly shaped ion beams in the keV energy range with constant radial current density across a large part of the beam diameter. The device consists of an ion source, a cylindrical extraction cup electrode and a cylindrical acceleration tube electrode, all disposed on the same axis. Positive ions are axiallyextracted from the ion source by the extraction electrode which is kept at a small negative potential (less than one tenth of the acceleration potential) with respect to the ion source. The extracted ions are accelerated and focused by an immersion lens which is formed by the potential distribution within and between the extraction and acceleration electrodes, the acceleration electrode being at such a negative potential with respect 'to the ion source that the focused ions possess the desired energy after having passed through the immersion lens. Under observance of well-defined restrictive conditions for the lay-out of several geometrical dimensions of the construction and under observance of welldefined restrictive operation conditions of the ion gun it is possible to obtain circularly shaped ion beams with an approximately trapezoidal radial current density distribution at axial target positio' far removed from the acceleration field. 1 1

9 Claims, 4 Drawing Figures Patent Oct. 7,1975 S11eet10f3 3,911,314

FIG. I 2? FIG. 3

US. Patent 0a. 7,1975 Sheet 2 of 3 3,911,314

FIG. 2

\ H 1 r \i|ll b o0 2 v llllll-|n. B 4 W/ a 1 J U.S. Patent 0117,1975 616113013 3,911,314

J! I A V A m 1 0 w 4 7 5 4 3 2 .7 EE\ l\ 5 2m *cmtau Radial dis tance [mm] ION GUN FOR PRODUCTION OF ION BEAMS WITI-I PARTICULAR RADIAL CURRENT DENSITY PROFILE CROSS REFERENCE TO RELATED APPLICATION The present application is a continuation-in-part of our copending application Ser. No. 356,314, filed May 2, 1973, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to ion guns and more particularly to an improved ion gun with a two step acceleration system.

Ion guns have been constructed so far mainly with the aim of producing high current ion beams of high brightness. The problems occurring in design and operation of the respective ion source have recently been reviewed by A. Septier in a paper in the book Focusing of Charged Particles, Vol. II edited by A. Septier, published 1967 by Academic Press.

In many applications of ion irradiation such as sputtering and ion implantation it would be desirable to bombard the target with a beam of constant current density across the total area to be covered. Such a homogenous irradiation of large areas has been achieved so far by scanning a focused beam. Besides the additional expenditure needed to use this technique the principal drawback of this method is that during bombardment no equilibrium between sputtering and formation of surface layers of residual gas can be achieved because each element of the bombarded area is hit by the beam only intermittently. In some cases an approximately homogeneous irradiation was obtained by strongly defocusing the beam at the target position. This method has the drawback that most of the ion current originally produced does not hit the target and is thus wasted.

In several investigations concerning the current density distribution of ion beams (beam profile) it was found that nearly all operation parameters of the ion gun contribute to the respective shape of the distribution. These effects have been described, e.g. by W. L. Rautenbach in a paper published in Nuclear Instruments and Methods 12 1961 and by us, K. Wittmaack and F. Schulz, in the Proceedings of the International Conference, Electron and Ion Beam Science and Technology, edited by R. Bakish, published in 1972 by the Electrochemical Society. The variety of beam profiles observed led to the conclusion that under well-defined restricted operation conditions and by observance of additional precautions with respect to the geometrical lay-out of certain construction elements of an ion gun it might be possible to achieve ion beams with constant current density acrossa large part of the beam diameter. I

SUMMARY'QOF THE INVENTION formation. If one keeps the diameter of the ion source opening and the diameter of the extraction and acceleration electrodes fixed,.either for reason of a certain maximum allowable gas flow out of the source or because of limitations with respect to the dimension of the complete gun, then one would like to vary: (i) the length of the ion source opening, (ii) the diameter of the extraction electrode aperture facing the ion source opening, (iii) the length of the extraction electrode, (iv) the extraction gap distance and (v) the acceleration gap distance.

In a preferred embodiment the dimensions are variable either in situ or after dismounting part of the ion gun. Detailed investigations have shown that to obtain trapezoidal beam profiles restrictions with respect to dimensions (i) through (v) have to be obeyed. Moreover there are restrictions with respect to the operation conditions of the ion gun comprising the gas flow and the plasma density in the ion source and the magnetic field strength.

The main features of our invention thus lie in the finding that under well-defined conditions with respect to the lay-out of an ion gun of basically known design and with respect to the operation of the gun, trapezoidal current density profiles can be obtained.

The invention itself may best be understood by reference to the following description taken in conjunction with the following drawings:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a transverse sectional view of the preferred embodiment of the improved ion gun.

FIG. 2 is a schematic representation of the main part of the ion gun indicating all dimensions discussed.

FIG. 3 is a graph showing the relation between extraction gap distance d and reduced length of the extraction electrode, L/D, to be obeyed for production of trapezoidal beam profiles.

FIG. 4 is a graph illustrating typical beam profiles at different extraction gap distances.

DETAILED DESCRIPTION OF THE PREFERRE EMBODIMENT Referring now to FIG. 1, and partly also to FIG. 2, there is shown an ion source I with a filament 15 for thermal emission of electrons and a coil 19 for production of an electron confining magnetic field B (see FIG. 2). The ions can leave the ion source 1 only via the axially disposed ion source outlet 16. The ion source 1 is mounted on the ion source support flange 2 which itself is fixed to the extractor support flange 3. The cylindrical cup extractor or extraction electrode 4 is press fitted to the cylindrical support electrode 20 which itself is held in place by three insulating screws 18 engaging in corresponding threads in electrode 20. The screws 18 are axially fixed to the extractor support flange 3 but can be rotated during operation of the ion gun by manipulations from the outside of the gun vessel. This is achieved by rotating three insulating shafts 25. The rotary motion of the shafts 25 is transmitted to the screws 18 via bevel gears and three rotary shaft vacuum seals 5. The rotary motion of the screws 18 causes an axial motion of the extractor 4 with respect to the ion source 1 (and the cylindrical tube acceleration electrode 7) thus allowing a variation of the extraction gap distance d (see FIG. 2). The potential of the extraction electrode 4 is set by a voltage applied thereto via a conductor passing through insulator 6 and a lead 21.

Exit opening 13 of extraction electrode 4 and entrance opening 12 of the acceleration electrode 7 are positioned to face one another. The length of either electrode and the acceleration gap distance g (see FIG. 2) may be varied by replacing intermediate rings 22 and 23 after dismounting the ion source support flange 2 together with the ion source 1. The accelerating electrode 7 is mounted via the intermediate rings 22 and 23 and the cylindrical funnel electrode 24 to the acceleration electrode support flange 8 which contains the acceleration electrode exit opening 11.

A glass tube 9 with end surfaces 10 and 14 serves simultaneously a vacuum vessel and as a means to provide insulation between extractor support flange 3 and acceleration electrode support flange 8. Flange 8 can be mounted to conventional vacuum vessels and vacuum pumps.

Referring now in more detail to FIG. 2, there are shown schematically the main elements of the preferred embodiment shown in FIG. 1, with the aim of designating the critical dimensions of the construction. The ion optically relevant lengths are the diameter of the extraction electrode, D,, and the diameter of the acceleration electrode, D Therefore most of the axial dimensions are given in these units, e.g. the length of the extraction electrode 4, L=mD and the distance, Q=nl) between a target 27 on target holder 26 and the midplane 28 between the exit opening 13 of the extraction electrode 4 and the entrance opening 12 of the acceleration electrode 7. Typically, n 5. To measure the current density profile of the ion beam, target 27 and target holder 26 are replaced by a suitable monitor.

ln test modes the ion source 1 was run with argon a working gas, the argon pressure in the source being in the range 3X10 to 3X10 Torr, the residual gas pressure in the vacuum vessel being about 2X10 Torr. The argon ion energy was varied between 10 and 50 keV. The magnetic field B in the ion source 1 was varied between zero and 600 Gauss. The following dimensions were kept fixed:

a= 1.5 mm

II 1.0 mm

1), D 1)= 46 mm Q= D.

1f no conditions, or limits, are placed on the values of the other dimensions shown in FIG. 2, the resulting beam will normally have a Gaussian, or bell-shaped, radial intensity distribution, as represented by the broken-line curve in FIG. 4, representing the beam current density at a target surface as a function of the radial distance from the beam axis. It has been found that a trapezoidal radial intensity distribution, represented by the solid-line curve in FIG. 4, can be achieved only if the dimensions 1, b and g are given values within certain ranges, as follows:

0.5 s 1 s 1.5 mm

2.0 s b s 4.0mm

These ranges are not substantially influenced by the value of the extraction electrode length, L.

In addition, to achieve such trapezoidal intensity distribution, the dimension d must be given a value such that d=f(m); where NFL/I). The specific nature of this function is shown in FIG. 3, which presents a curve representing the average value of f( m) and a series of vertical bars illustrating the range over which d can vary at certain selected values of m. Thus, for example, for m=l .22, d must have a value of between approximately 2 and 2.5 mm in order for a trapezoidal current density distribution to be produced.

It can be deduced from the curve of FIG. 3 that f(m) (average) can be approximated by the expression K(m0.8), where K has a value between 10 and 15 and y has a value of roughly 2.

Additional restrictions were found to exist with respect to the operation conditions of the ion gun: (I) The magnetic field B in the ion source 1 should be larger than 50 Gauss. (2). The argon pressure in the ion source 1 should be less than 2X10 Torr, otherwise profile distortions will occur. (3) The total beam cur rent should not fall short of a lower limit or exceed an upper limit. These limits depend upon the ion species, the ion energy, the diameter of the ion source opening, a, and the diameters of extraction and acceleration electrode, D,, D In case of the above specified values for a, D, and D and for an argon ion energy of 40 keV, the two limits for the beam current are 20 ,uA and 120 [.LA, respectively. Below 20 ;/.A no pronounced plateau on the beam profile is developed, above 120 ,u.A lens defects lead to measurable profile distortions.

The curves shown in FIG. 4 represent current distributions obtained with the gun shown in FIGS. 1 and 2 and with an ion source employing argon, an ion energy of 40 keV, a beam current of ILA, an extractor length, L, of 1.22 D (m 1.22), an acceleration gap distance, g, of 0.22 D, an ion source outlet length, l, of 0.6 mm, an extraction electrode entrance aperture diameter, b, of 3 mm, an ion source outlet diameter, a, of 1.5 mm, an extractor entrance aperture length, 11, of 1.0 mm, extraction and acceleration electrode diameters, D, of 46 mm, and a target distance, Q, of 15 D. Only the extraction gap distance, d, was varied to produce the two current distributions shown in FIG. 4. When d had a value of 5 mm, outside the range indicated in FIG. 3 for L/D= 1.22, the current distribution was Gaussian. On the other hand, when d was set to a value of 2 mm, within the range indicated in FIG. 3, the current distribution was trapezoidal.

The effect of the above-cited values for dimensions,

I, b, g and d on the production of a trapezoidal current distribution'at a target plane is demonstrated by the following Table showing results obtained using an ion gun having the following parameters: L/D 1.0, a 1.5 mm, ion energy 40 keV, beam current 20-120 pA. The values presented in the Table were measured with the following accuracies: I: 0.1 mm; b i 0.1 mm; d i 0.2 mm; g 1mm.

TABLE 1 b d g Trapezoidal [mm] [mm] [mm] beam profile Remarks G l lmmI 0.1 2.5 20.5 no 0.6 2.5 0.5-l 10 yes 0.6 2.5 21.5 10 no 1.0 2.5 0.5-1 10 yes 1.0 2.5 21.5 10 no 1.5 2.5 0.5-l 10 yes 1.5 2.5 21.5 10 no 2.2 2.5 20.5 10 no 0.6 1.5 21 10 no 0.6 3.0 1 10 yes 0.6 5.0 20.5 10 no 0.6 2.5 -l 10 yes Profile width 0.6 2.5 -l 25 yes decreasing with 0.6 2.5 -l yes increasing g It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

We claim:

1. In an ion gun for production of ion beams with particular current density profile at a target plane intersected by the ion beam axis, the gun including: an ion source of the magnetically confined electron bombardment type with hot cathode, oscillating electrons and ion extraction parallel to the magnetic field through an axially disposed ion source outlet; a cylindrical extraction cup electrode with an entrance aperture facing said ion source outlet and an exit opening opposite to said entrance aperture, the extraction electrode being axially disposed with respect to said ion source, and the target plane being located beyond the extraction electrode; and a cylindrical acceleration tube electrode with an entrance opening facing said exit opening of said extraction electrode, the acceleration electrode being axially disposed with respect to said ion source and said extraction electrode; the improvement wherein: the outlet of said ion source has a length which is smaller than its diameter; the entrance aperture of said extraction electrode has a diameter which is larger than the diameter of the outlet of said ion source; the axial distance between the outlet of said ion source and said extraction electrode entrance aperture is a function of the ratio of the axial length to the inner diameter of said extraction electrode; the axial distance between said extraction electrode and said acceleration electrode is less than the inner diameters of said extraction and acceleration electrodes; and the axial distance between the target plane and the midplane between said extraction and acceleration electrodes is more than 5 times the inner diameters of said extraction and acceleration electrodes; whereby, the resulting current density profile has a trapezoidal form at the target plane.

2. The device of claim 1 wherein said extraction electrode and acceleration electrodehave equal inner diameters.

3. The device of claim 2 wherein the axial magnetic field strength in the ion source exceeds 50 Gauss.

4. The device of claim 3 wherein the pressure of the working gas in the ion source does not exceed the minimum pressure by more than a factor of five.

5. The device of claim 1 wherein the axial length of the outlet of said ion source is between 0.5 and 1.5 mm; and the diameter of said extraction electrode entrance aperture is between 2 and 4 mm.

6. The device of claim 5 wherein said ion source contains argon as its working gas, at a pressure of less than 2 X 10' Torr.

7. The device of claim 5 wherein said ion source produces a beam constituted by a current of between 20 and pA.

8. The device of claim 1 wherein the axial distance between the outlet of said ion source and said extraction electrode entrance aperture equals K(m0.8 where K has a value of between 10 and 15 mm, y has a value of approximately 2, and m is the ratio of the axial length to the inner diameter of said extraction electrode.

9. The device of claim 1 wherein the axial length of said entrance aperture of said extraction electrode is less than two times the diameter of said aperture. 

1. In an ion gun for production of ion beams with particular current density profile at a target plane intersected by the ion beam axis, the gun including: an ion source of the magnetically confined electron bombardment type with hot cathode, oscillating electrons and ion extraction parallel to the magnetic field through an axially disposed ion source outlet; a cylindrical extraction cup electrode with an entrance aperture facing said ion source outlet and an exit opening opposite to said entrance aperture, the extraction electrode being axially disposed with respect to said ion source, and the target plane being located beyond the extraction electrode; and a cylindrical acceleration tube electrode with an entrance opening facing said exit opening of said extraction electrode, the acceleration electrode being axially disposed with respect to said ion source and said extraction electrode; the improvement wherein: the outlet of said ion source has a length which is smaller than its diameter; the entrance aperture of said extraction electrode has a diameter which is larger than the diameter of the outlet of said ion source; the axial distance between the outlet of said ion source and said extraction electrode entrance aperture is a function of the ratio of the axial length to the inner diameter of said extraction electrode; the axial distance between said extraction electrode and said acceleration electrode is less than the inner diameters of said extraction and acceleration electrodes; and the axial distance between the target plane and the midplane between said extraction and acceleration electrodes is more than 5 times the inner diameters of said extraction and acceleration electrodes; whereby, the resulting current density profile has a trapezoidal form at the target plane.
 2. The device of claim 1 wherein said extraction electrode and acceleration electrode have equal inner diameters.
 3. The device of claim 2 wherein the axial magnetic field strength in the ion source exceeds 50 Gauss.
 4. The device of claim 3 wherein the pressure of the working gas in the ion source does not exceed the minimum pressure by more than a factor of five.
 5. The device of claim 1 wherein the axial length of the outlet of said ion source is between 0.5 and 1.5 mm; and the diameter of said extraction electrode entrance aperture is between 2 and 4 mm.
 6. The device of claim 5 wherein said ion source contains argon as its working gas, at a pressure of less than 2 X 10 3 Torr.
 7. The device of claim 5 wherein said ion source produces a beam constituted by a current of between 20 and 120 Mu A.
 8. The device of claim 1 wherein the axial distance between the outlet of said ion source and said extraction electrode entrance aperture equals K(m-0.8)y, where K has a value of between 10 and 15 mm, y has a value of approximately 2, and m is the ratio of the axial length to the inner diameter of said extraction electrode.
 9. The device of claim 1 wherein the axial length of said entrance aperture of said extraction electrode is less than two times the diameter of said aperture. 